Fever as a Symptom of Cancer

Wife taking hudband'’s temperature
Fever as a Symptom. Paul Bradbury / Getty Images

Let’s say you’re otherwise healthy, it’s in the middle of flu season, and you develop a fever along with a sore throat, nasal congestion, cough, headache, and fatigue. And your child was sick with the flu last week.

Leukemia and lymphoma are not likely to be on your doctor’s short list of diagnoses for you since you probably have the flu.

But sometimes a fever, whether alone or in combination with other symptoms, is a part of something else, and your doctor is trained to pick up on clues that indicate the need for a closer look. The vast majority of fevers seen by doctors can be traced to non-malignant causes.

Fever of Unknown Origin

In the medical community, FUO stands for “fever of unknown origin.” Having FUO does not mean you have cancer. In fact, there are a huge number of noncancerous causes of FUO.

But as some people with leukemia or lymphoma can attest to, a fever that wouldn’t go away—maybe together with fatigue and a lump—was how it all began, for them.

FUO is not just any old fever that lacks an obvious cause. In FUO, the fever has to be prolonged, and there has to be a pretty extensive medical workup to rule out numerous likely causes before a fever can be labeled a fever of unknown origin.

FUO is defined as “a fever of 38.3 C (101 F) or more, lasting for at least three weeks, for which no cause can be identified after three days of investigation in the hospital or after three or more outpatient visits.” The length of time in this definition conveniently tends to get rid of some of the numerous, more common causes of fever that resolve within three weeks.

What Causes FUO?

Unfortunately for doctors and patients, the list of possible causes—even for fevers lasting longer than 3 weeks—is quite long.

Statistically, the breakdown of likely causes depends on things like your geography (where you live in the world) and your demographics (for instance, whether you are a child or a grown-up). A rough breakdown of causes for U.S. adults follows:

  • Infection accounts for about 15-25 percent of cases of FUO.
  • Malignancies, autoimmune and/or rheumatologic diseases, and a long list of miscellaneous causes account for the remainder of identified causes.
  • No cause can ever be found in about 20 to 50 percent of cases.

The most common malignancies for which fever is an early sign include lymphoma (especially non-Hodgkin) and leukemia.

Why do Blood Cancers Cause Fevers?

Weight loss, fatigue, and fevers may all go together in the case of cancer, and two kinds of blood cancer in particular—leukemia and lymphoma—are known to produce fevers.

While infection is always a possible cause for a fever, it’s believed that, in some cases of leukemia and lymphoma, the malignant cells, themselves, may produce chemical signals that cause the body to elevate the core temperature.

When blood cancers do cause fevers, those fevers can in some cases impact the stage and prognosis or outlook of the illness. In some blood cancers, the presence of fever, night sweats, and unintentional weight loss suggests that the cancer is more advanced and that more intensive treatment might be required.

What Else Can Cause FUO?

Putting infection, malignancy, and autoimmune and rheumatologic disorders aside, a long list of relatively rare causes of FUO remains.

Sometimes drugs may be to blame, including potentially certain antibiotics, as well as medications that are taken to prevent seizures, and even pain medications like NSAIDs.

Sometimes the source of the fever actually is an infection, but one that got overlooked by doctors and patients alike, such as a dental infection with an abscess. FUO may be HIV-associated or observed along with a compromised immune system. Still, other possibilities include alcoholic hepatitis and blood clots in the deep veins (deep venous thrombosis).

A Word From Verywell

Although blood cancers like leukemia and lymphoma do often present with a fever that won’t go away, it’s important to work with your doctor to rule out more obvious causes, such as infection, and to consider your entire clinical picture along with your doctor, including other signs or symptoms that would make a different diagnosis much more likely.

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